Many sources of energy that can be harvested are in the form of electricity or can be readily converted to it. However in most practical situations, these electricity sources are weak, with too high or too low a voltage, and/or with certain other issues such as low duty cycle. Consequently, they are rarely directly useful to any electronic devices, which normally require a relatively steady supply of a direct current (DC) voltage of a few volts.
There is a need for simple and practical ways to efficiently convert the electricity generated by low-power energy harvesting systems of intermittent high voltage and low current, such as tribo- or piezo-electricity generators, into a usable form for low-power electronic devices such as those in the Internet of Things (IoT) applications.
In general, to turn the electricity generated by low-power energy harvesting systems of high voltage and low current, such as tribo-/piezo-electricity, into a usable form, a step-down DC-DC converter is used to turn a high voltage and low current DC into another DC with a lower voltage and a larger current.
There are many step-down DC-DC converters available on the market. Some of them use capacitors and inductors for energy transfer and some are inductor-free such as those using the charge pump and switched-capacitor principles.
In many available low-power energy harvesting systems of intermittent high voltage and low current such as those for tribo-/piezo-energy harvesting, the harvested electricity is alternating current (AC) and can have an open-circuit peak voltage of 200-300 volts or beyond and a corresponding short-circuit current of a few tens of micro-amps. Nevertheless, the average current is substantially lower given the extremely low duty cycle of the peaks. Since the tribo-/piezo-electricity in most practical cases is weak, fluctuating a lot and even intermittent, the existing step-down DC-DC converters have the following limitations if used in this energy harvesting application:                their input needs to be a steady DC in a relatively small range (a few volts to a few tens of volts) in order for them to work properly, while the voltage from tribo-/piezo-energy harvesting devices most likely fluctuates a lot and even disappears;        their input voltage should be no more than a few tens of volts, whereas the voltage in the tribo-/piezo-electricity realm is easily 200-300 volts or beyond; and        they themselves consume significant amount of power (a few μA to a few tens of μA), known as the overhead. Consequently,        i. most tribo-/piezo-energy harvesting devices cannot afford to provide this overhead power; and        ii. even if in quite unlikely cases these devices can provide the overhead power, the DC-DC converter's efficiency will be too low to be practical when operating under such low-power scenarios.        
Some existing step-down DC-DC converters employ a MOSFET as the switch. While this seems to be a viable option from a first glance, there are practicality issues associated with the use of a MOSFET as the switch. These issues are:                Usually, a high voltage compatible MOSFET when off has a relatively large leakage current, about a fraction of 1 ρA. Although this may be acceptably small in most other applications, it may not be ignored in the context of tribo-/piezo-energy harvesting, where the average current an energy harvesting device is able to produce is in the same order if not less. As a result, at least a significant portion of the harvested energy will be wasted due to the MOSFET leakage.        A MOSFET needs to be driven by a control module, consisting of maybe a microcontroller system and a driver capable of producing a few volts and a significant current—to overcome the gate-source turn-on voltage and the gate capacitance, respectively. Such a need complicates the system design and implementation and may add additional constraints if not done properly.        The need for a control module imposes another potential issue: the system may not be able to self-start when there is no auxiliary power available, e.g., a battery or the like.        